The present invention relates to improvements in or relating to telecommunication systems and is more particularly concerned with lowering the bit rate required for speech transmission in such systems.
Telecommunications systems include both xe2x80x98wiredxe2x80x99 systems, for example, standard telephony land lines, and xe2x80x98wirelessxe2x80x99 systems, for example, mobile or cellular systems, and each system requires that the bit rate for speech transmission be lowered to optimise its transmission carrying capacity. For example, a mobile system comprises a plurality of cells, each cell being defined by at least one base station and a plurality of mobile terminals and having a predetermined maximum capacity. When a connection is made between a pair of terminals, namely, a transmitting terminal and a receiving terminal, speech to be transmitted is input at a microphone in the transmitting terminal as an analogue signal. The analogue signal is digitised to provide a digital signal which has a predetermined bit rate, typically, 64 kbit/s. The digital signal is then passed to a speech coding device which analyses the sounds in the signal and codes it for transmission on a predetermined channel to a receiver in the receiving terminal. Typically, the bit rate for the transmission is 8 kbit/s. The receiver decodes the transmitted signal and converts it to an analogue signal which can then be broadcast through a speaker in the receiving terminal. The coded digital signal is normally transmitted from terminal to terminal via a base station.
When a plurality of terminals are transmitting at the same time in a cell, the cell may approach its predetermined maximum capacity and, as a result, the number of connections in that cell may be restricted. Furthermore, or a transmission bit rate of 8 kbit/s, relatively high bandwidth channels may be required.
It is therefore an object of the present invention to provide a method of reducing the bandwidth requirements for speech transmission.
In accordance with one aspect of the present invention, there is provided a method for reducing transmission bit rate in a telecommunications system, the method comprising the steps of:
a) receiving a signal at a microphone;
b) converting the received signal to a coded signal for transmission;
c) recognising speech; and
d) replacing parts of the coded signal with codewords representative of the recognised speech.
In one embodiment, steps b) and c) are carried out simultaneously. In another embodiment, step c) is carried out prior to step b). In a further embodiment, step b) is carried out prior to step c).
Advantageously, increased battery life may be achieved if a discontinuous transmit mode is used, lower bandwidth channels may be utilised, and higher cell capacity may be achieved. This is particularly true for code division multiple access (CDMA) systems.
The term xe2x80x98recognised speechxe2x80x99 is intended to encompass not only words, phrases or sentences, but sounds which make up parts of words.